This invention relates to electro-chemical fuel cell systems, and, more particularly, to the integrated pressurization and moisturization of the fuel cell.
During the course of the 20th Century, science and engineering have brought us to an era whereby every American enjoys the availability of abundant energy as well as advanced technology for heating and air-conditioning of homes, transportation, and industry. Nevertheless, these wonders of modern society have come at a severe price to our global environment and to our reserves of nonrenewable natural resources. Fossil fueled electrical power generation and automobiles have increased the level of carbon dioxide in the atmosphere to the point where deleterious global warming effects on the environment are predicted. Furthermore, due to the release of chlorofluorocarbons (CFC""s) from conventional refrigeration and air conditioning systems, the ozone layer protecting the earth from deadly ultraviolet radiation is being depleted, with serious projected consequences for mankind. This crisis is considered so serious that 159 nations, including the United States, met in Kyoto, Japan in December of 1997 and signed a treaty designed to limit the buildup of carbon dioxide and other greenhouse gases in our environment. Immediately following the signing of the treaty, there was an outpouring of concern that meeting the targets outlined in Kyoto would be impossible without either modifying radically the American way of life, or of finding new energy-efficient and non-polluting technologies.
According to xe2x80x9cFuel Cellsxe2x80x9d by McDougall, John Wiley and Sons, 1976, page 78, the first successful working fuel cell was produced by F. T. Bacon in 1957. This fuel cell had a working temperature of 200xc2x0 C. and a pressure of 20-40 atm. For all fuel cells, the well known Nernst equation predicts that the output voltage of a fuel cell should increase with the partial pressures of fuel and oxygen. For many types of fuel cells, the power output and the efficiency of the fuel cell stack generally increases with pressure as well. For example, the xe2x80x9cFuel Cell Handbookxe2x80x9d by Appleby, Van Nostrand-Reinhold, 1989, discusses the improvements in power output and efficiency obtained by pressurizing various types of fuel cells including the Phosphoric Acid Fuel Cell (PAFC), the Molten Carbonate Fuel Cell (MCFC), and the Solid Oxide Fuel Cell (SOFC). However, in all cases, this improvement in performance must be paid for by the energy input required to compress the reactants and the increased complexity of the system. In theory, an ideal compressor-expander can pressurize the fuel cell with a minimal energy requirement if the compressor and expander operate at 100% efficiency. In such a system, the fuel cell is pressurized on the air side by a compressor, and the oxygen depleted exhaust gases energize an expander which drives the compressor, requiring very little additional energy input. In practice, component efficiencies become very crucial, and substantial amounts of external energy must be supplied, thereby lowering the improvement in overall system efficiency gained through pressurization. With current technology, this can involve very expensive and bulky machinery. For this reason, in the current state-of-the-art, PAFC""s, MCFC""s, and SOFC""s, are normally operated at atmospheric pressure.
While the advantages and disadvantages of pressurization apply to all fuel cells, they have recently become of prime importance in the Department of Energy xe2x80x9cPartnership for a New Generation of Vehiclesxe2x80x9d (PNGV) program in fuel cells for automobiles where the goal is to obtain a vehicle which will achieve 80 miles/gallon of fuel. Current emphasis in the PNGV fuel cell vehicle program is to use the Proton Exchange Membrane Fuel Cell (PEMFC) since this type offers reduced weight and size, faster start-up, operation at room temperature, and potentially lower cost. Such a fuel cell is normally pressurized to about 3 atmospheres on the air side, and with hydrogen rich gas produced by an on-board gasoline reformer. Kumar (U.S. Pat. No. 5,248,566) calls for PEMFC operating pressures of about 2-5 atmospheres on the hydrogen side. With PEMFC""s in the automotive environment, pressurization is particularly important since the size of the stack must be minimized while the power output maximized. Also, since the PEMFC has a solid fluorocarbon electrolyte which must be humidified in order to function, a sufficiently high water vapor pressure without diluting the hydrogen and oxygen can only be obtained if the system is pressurized. Thus, a critical technology in the success of a fuel-cell-powered automobile is a high efficiency compressor-expander, capable of a wide dynamic range of operation, low in cost, low in volume and weight, and capable of humidity control. An objective of the present invention is to provide a system which can meet these requirements.
In the development of new technologies which will enable us to continue to enjoy our prosperity yet preserve the environment, there has been a profound need for high efficiency compressor-expanders for pressurizing fuel cells. This is an area of technology whereby improvements can have a major global impact on the amount of the energy we consume and the pollution we create, particularly with regard to greenhouse gases and ozone layer depleting chemicals. fluid at the discharge of the nozzle is substantially reduced.
This disclosure further provides a pressurization system for fuel cell power plants which is particularly useful for automotive applications using PEM fuel cells. The disclosure recited herein has a compressor-expander which is powered by exhaust gases from the fuel cell and discharges said exhaust gas to the atmosphere. Said compressor-expander receives clean secondary air from the ambient and compresses it for discharge into the secondary inlet of an ejector. The primary of said ejector is steam provided from a boiler. The steam provides the excess energy needed to make up for inherent losses, and it also provides needed moisture for the fuel cell. The disclosed system can be more compact than comparable compressor-expander-motor combinations utilizing conventional machinery.